From bf7e07d299b197891110fbd8c717badbab06a472 Mon Sep 17 00:00:00 2001
From: Ludovic Courtès <ludo@gnu.org>
Date: Fri, 1 Jul 2022 16:29:53 +0200
Subject: services: openssh: Listen on IPv6 only when IPv6 is supported.
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Fixes <https://issues.guix.gnu.org/56327>.
Reported by André Batista <nandre@riseup.net>.

* gnu/services/ssh.scm (openssh-shepherd-service)[ipv6-support?]: New
variable.
Use it in 'start' method.
---
 gnu/services/ssh.scm | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/gnu/services/ssh.scm b/gnu/services/ssh.scm
index 57d3ad218c..72e7183590 100644
--- a/gnu/services/ssh.scm
+++ b/gnu/services/ssh.scm
@@ -536,6 +536,15 @@ of user-name/file-like tuples."
     #~(and (defined? 'make-inetd-constructor)
            (not (string=? (@ (shepherd config) Version) "0.9.0"))))
 
+  (define ipv6-support?
+    ;; Expression that returns true if IPv6 support is available.
+    #~(catch 'system-error
+        (lambda ()
+          (let ((sock (socket AF_INET6 SOCK_STREAM 0)))
+            (close-port sock)
+            #t))
+        (const #f)))
+
   (list (shepherd-service
          (documentation "OpenSSH server.")
          (requirement '(syslogd loopback))
@@ -544,12 +553,15 @@ of user-name/file-like tuples."
          (start #~(if #$inetd-style?
                       (make-inetd-constructor
                        (append #$openssh-command '("-i"))
-                       (list (endpoint
+                       (cons (endpoint
                               (make-socket-address AF_INET INADDR_ANY
                                                    #$port-number))
-                             (endpoint
-                              (make-socket-address AF_INET6 IN6ADDR_ANY
-                                                   #$port-number)))
+                             (if #$ipv6-support?
+                                 (list
+                                  (endpoint
+                                   (make-socket-address AF_INET6 IN6ADDR_ANY
+                                                        #$port-number)))
+                                 '()))
                        #:max-connections #$max-connections)
                       (make-forkexec-constructor #$openssh-command
                                                  #:pid-file #$pid-file)))
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